It is well known that hydrocarbon oils containing an appreciable concentration of materials boiling above about 1050.degree. F. are difficult to process in conventional FCC operations because these feeds contain appreciable concentrations of materials which both temporarily and permanently impair the effectiveness of conventional zeolitic cracking catalysts. These impurities include: asphaltenes (Conradson carbon) which deposit on the catalyst particles to form coke, frequently in an amount in excess of that which can be tolerated by an existing FCC regeneration system; metals, especially nickel and vanadium, usually at least partially in the form of porphyrins, which are frequently referred to as catalyst poisons and which build up on catalyst particles during reaction/regeneration cycles to levels necessitating undesirably high fresh catalyst replacement levels; and nitrogenous bases which interfere which acidic cracking sites of the zeolite component of the catalyst during the cracking cycle. Exemplary of such impure oils are atmospheric and vacuum residual oils (resids), tar sand oils as well as clean gas oils blended with resids or other impure oils. Even clean gas oils contain deleterious nitrogenous bases. Sodium in feedstocks or introduced in steam used in FCC processing is also harmful to catalytic cracking.
Staged processing in separate process steps is old in the catalytic cracking art. It has been proposed, for example, to add to a conventional cyclic FCC operation a vapor/solid pretreatment stage to reduce the content of impurities in oil feedstocks before the oils are cracked catalytically. In particular, it has been proposed to remove the impurities by selectively vaporizing the valuable high hydrogen components of the oil by contacting the oil with hot particles of sorbent particles, such as microspheres of calcined clay, leaving carbonaceous, metals, nitrogenous and sulfurous impurities present as a deposit on the particles of sorbent contact material. Proposed equipment takes advantage of the fast fluid riser type of equipment used in FCC units, namely, a riser in which selective vaporization and impurity removal takes place by dilute phase ultrashort contact between feed and hot contact material and a regenerator (burner) in which coke is burned from the impurity-laden particles of contact material, thereby renewing the activity of the contact material and supplying the heat needed by the particles to vaporize incoming charge of hydrocarbon feed to the riser. The sorbent particles used in the process have a low surface area, typically below 10 m.sup.2 /g by the BET method, and are essentially devoid of catalytic cracking activity. Such cracking that does take place is largely of thermal character. Since the vaporization takes place in a fast fluid riser, contact between hydrocarbon and sorbent is short, about 2 seconds or less, and little undesirable recracking of vapors takes place in the riser. In a further attempt to avoid recracking, the vapors and particles of sorbent are rapidly separated from each other and the separated vapors are quenched prior to being charged to the FCC unit. This type of process, referred to commercially as the ART process, is described in numerous publications and patents, exemplary of which are: U.S. Pat. No. 4,263,128 (Bartholic), U.S. Pat. No. 4,781,818 (Reagan et al.), and "The ART Process Offers Increased Refinery Flexibility," R. P. Haseltine et al., presented at the 1983 NPRA Conference in San Francisco.
In an embodiment of the pretreatment processing scheme described above, the vapors from the selective vaporization step, after removal of spent sorbent particles therefrom, are charged directly to an FCC unit without prior quenching. See U.S. Pat. No. 4,525,268 (Barger, et. al.)
A characteristic of these pretreatment processing schemes is that selective vaporization with associated impurities removal and cracking take place in different units and regeneration of contact sorbent and cracking catalyst also takes place in different units. Thus, particles of zeolite cracking catalyst and sorbent particles are never intentionally commingled during the cyclic process. In fact, the zeolitic catalyst particles and sorbent particles are intentionally isolated from each other and only an upset in a unit operation results in commingling of catalyst and sorbent. The practice of maintaining isolation of sorbent and catalyst particles is dictated in part by the intent to avoid contamination of zeolitic catalyst particles during the cracking cycle with impurities picked up from the oil and deposited on the sorbent particles and in part by the need to use separate regenerators to avoid undesired contamination of the catalyst with metals and nitrogenous bases as a result of migration from the sorbent during high temperature regeneration. Furthermore, the regeneration requirements are generally different for the two different classes of coked materials because of the difference between the nature of the coke on the sorbent and catalyst particles. Regenerators for the sorbent usually require higher temperature regeneration than is needed to regenerate catalyst particles. The temperatures needed to burn the relatively high hydrogen content coke deposit on sorbent particles may result in the destruction of the zeolitic component catalyst particles and/or result in overcracking of feedstock.
The following relate to staged contacting in FCC or other catalytic cracking operations:
U.S. Pat. No. 2,472,723, (Peet), U.S. Pat. No. 2,956,004, (Conn, et. al.) and U.S. Pat. No. 3,146,188, (Gossett) describe discrete staged treating process for upgrading heavy feeds.
U.S. Pat. No. 3,639,228, (Carr, et. al.) and U.S. Pat. No. 4,257,875 (Lengemann, et. al.) describe staged contacting using a single riser and a single regenerator, but utilizing only one type of catalyst.
U.S. Pat. No. 2,943,040, (Weisz) discloses catalytic cracking processes using a mixture of catalysts of different particles sizes, one of which is an absorbent for metal and is introduced into a cracking process which may be fluidized. The absorbent is concentrated at one end, i.e., see col. 1, line 60 and following. The absorbent need not have catalytic cracking activity, i.e., col. 1, line 66. The patent does not teach the use of a riser or the staged regeneration contemplated by the present invention.
U.S. Pat. No. 4,416,814, (Zahner) relates to the use of two separate reactors with segregated feeds employing a single regenerator and two solids which may or may not be the same type but which are of different sizes.
In U.S. Pat. No. 4,525,268, (Barger), (discussed supra), staged contacting is practiced, but both segregated reactors and regenerators are utilized.
Pilot plant demonstrations of discrete two-stage treatment from three different crude oils are described in "Two Stage Non-Hydrogenative Processing of Residue," Krishna, AS. and Both, D. J.; 1. E. C. Proc. Des. Dev. 1985, 24, 1266-1275.
In U.S. Pat. No. 4,090,948 (Schwarzenbek) recycled spent (coked) cracking catalyst vaporizes feed in a lower zone of a riser in which vaporized feed is subsequently contacted with a recycled regenerated catalyst. Stripped spent catalyst is separated into two portions, one of which is recycled without regeneration to the lower zone of the riser and the other is recycled to an intermediate point in the riser.
Staged regeneration of spent fluid cracking catalysts with initial low temperature regeneration followed by high temperature full regeneration to control undesirable metal effects of high temperature is known in the art. See, for example, U.S. Pat. No. 2,943,040, (Weisz).
Other prior art includes:
U.S. Pat. No. 2,541,077, (Leffer)
U.S. Pat. No. 4,071,436, (Blanton, Jr., et. al.)
U.S. Pat. No. 4,116,814, (Zahner)
U.S. Pat. No. 4,243,556, (Blanton, Jr.)
U.S. Pat. No. 4,469,588, (Hettinger, Jr., et. al.)
U.S. Pat. No. 4,495,304, (Yoo, et. al.)
U.S. Pat. No. 4,569,754, (Moore)
U.S. Pat. No. 4,606,813, (Byrne, et. al.)
U.S. Pat. No. 4,655,905, (Plumail, et. al.)
U.S. Pat. No. 4,657,664, (Evans, et. al.)
U.S. Pat. No. 4,728,417, (Aldag, Jr. et. al.)
U.S. Pat. No. 4,729,826, (Lindsay, et. al.)
While it is well know that by incorporating a discrete sorption step upstream of the catalytic cracking step, improved activity and higher selectivity to desired products can be effected in the cracking operation, the known processing has involved the integration of separate processing steps. In many cases, the potential capital and operating steps upstream of the catalytic cracker would have more than offset the credits in the cracker.
One object of the present invention is to minimize the capital and operating expenses of staged processing, preferably within existing catalytic cracking unit designs with a minimal revamp, to provide for separate addition of sorbent solid and cracking catalyst to the same riser reactor, separation of sorbent from catalyst and segregated regeneration to fully burn coke from sorbent and catalyst particles under conditions appropriate for both so as to avoid transfer of potential catalyst poisons, especially metals, from the particles of sorbent to the particles of catalyst during regeneration.
The invention also provides a means for effectively increasing the throughput of existing catalytic crackers using conventional feeds such as clean gas oils and/or permits the economical processing of heavier feed.